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Aviano Air Base, Italy (CNN)Secretary of Defense Ash Carter announced Tuesday that Sirte, Libya, is now free of ISIS.

Carter called Sirte ISIS's "single most important nest and stronghold" in Libya. Carter described the US military operation of airstrikes against ISIS there as having "concluded."

A senior US defense official told CNN that ISIS has now lost all the territory it had controlled in Sirte. Libyan forces aligned with the US-backed government of national accord had for weeks struggled to drive ISIS from a small neighborhood in that coastal city with the help of US airstrikes.

In remarks at Aviano Air Base in Italy, Carter also announced the deaths of three ISIS leaders that were killed as the result of a coalition airstrike.

He said they were killed in one strike and were "linked to plots right here in Europe." Carter added, "We need to crush both the fact and the idea there can be an Islamic State."